1. Field of Invention
The present invention relates to a semiconductor device that has a circuit composed of thin film transistors (hereinafter referred to as TFTs) and a manufacturing method thereof, and for example, relates to an electronic device equipped with, as a component, an electro-optical device typified by a liquid crystal display panel or a light-emitting display device that has an organic light-emitting element.
It is to be noted that the semiconductor device in the specification refers to devices in general that can function by utilizing semiconductor characteristics, and electro-optical devices, semiconductor circuits, and electronic devices are all semiconductor devices.
2. Description of the Related Art
In these years, a technique of making thin film transistors (TFTs) by using a semiconductor thin film (on the order of several to several hundreds nanometers in thickness) formed on a substrate with an insulating surface has been attracting attention. The thin film transistors are widely applied to electronic devices such as ICs and electro-optical devices, and have been quickly developed in particular as switching elements for image display devices.
In particular, active matrix display devices (such as liquid crystal display devices or light-emitting display devices) in which a switching element of a TFT is provided for each of display pixels arranged in a matrix have been actively developed.
In active matrix display devices, developments for expanding effective screen regions in pixel portions have been advanced. In order to make the area of an effective screen region larger, it is really necessary to make the area occupied by TFTs arranged in a pixel portion (pixel TFTs) smaller as much as possible. In addition, in order to reduce manufacturing costs, developments for forming a driver circuit on the same substrate with a pixel portion have been also advanced. Above all, a TFT using a polysilicon film has a higher field-effect mobility than a TFT using an amorphous silicon film, and thus can operate at a higher speed.
In a module that is mounted on the display device, a pixel portion that displays images for each function block and a driver circuit for controlling the pixel portion, such as a shift register circuit, a level shifter circuit, a buffer circuit, and a sampling circuit are CMOS circuits in many cases, are formed on one substrate.
When a driver circuit and a pixel portion are formed on the same substrate, the area occupied by a region except a pixel portion, which is called a frame portion, tends to be larger as compared with mounting of a driver circuit by a TAB method. In order to make the area of the frame portion smaller, it is also really necessary to reduce the scale of circuits constituting the driver circuit.
In particular, for a light-emitting display device that has organic light-emitting elements (EL elements) arranged in a matrix, a plurality of TFTs that have different roles is required for one pixel. In addition, also for a liquid crystal display device, an attempt to form TFTs for switching and a memory element such as an SRAM in one pixel has been made. In addition, in the case of forming a pixel portion and a driver circuit on the same substrate, miniaturization as much as possible is desired.
In Japanese Patent Laid-Open No. 2001-51622, the use of a TFT that has a multi-gate structure (a structure that uses a semiconductor layer with two or more channel forming regions connected in series as an active layer) in an EL display device is described.
In addition, in Japanese Patent Laid-Open No. 2002-151523, a manufacturing process for a TFT is described in which a photomask or a reticle provided with an assist pattern that is composed of a diffraction grating pattern or a semi-transparent film and has a function of reducing a light intensity is applied to a photolithography process for forming a gate electrode.
In addition, in Japanese Patent Laid-Open No. 2002-203862, a manufacturing process for a TFT is described in which, with respect to a gate electrode that has a two-layer structure composed of different conductive materials, a first layer that is the lower layer of the gate electrode has a larger electrode width than a second layer of the gate electrode and doping a semiconductor layer through a portion of the first layer is carried out.